1. Field of the Invention
This invention relates to the preparation of semiconductor alloys having the general formula Hg.sub.(1-x) Cd.sub.x Te.sub.1 [where "X" is in the range of from about 0.14 to about 0.40 for infrared applications] generally referred to as "HgCdTe".
2. Description of the Prior Art and Background
Alloys containing mercury, cadmium and tellurium and having the general formula set forth hereinabove have been extensively reported in the prior art, particularly in conjunction with their semiconductor properties and their use in the area of infrared detectors and the like. A principal requirement of these materials is low concentration of impurities, the ideal concentration preferably being less than 10.sup.14 /cm.sup.3. It has, however, been difficult to obtain extrinsically doped P-type material with acceptor concentrations below about 10.sup.15 /cm.sup.3.
Fast diffusing acceptor impurities have been shown to preferentially segregate in regions of the mercury cadmium telluride sample which contain second phase tellurium. The area containing second phase tellurium is normally at the core or central region of the alloy slice. The reason is that, after a mercury cadmium telluride ingot has been formed in standard manner, it is then recrystallized and homogenized in a high temperature annealing step and then post-annealed at preferably low temperature for the normal post-annealing period of about one to four weeks. Such post-anneal normally results in an N-type skin free of excess Te and a core saturated with second phase tellurium. Mobile residual impurities in the alloy segregate to the "core" of the second phase tellurium at the center of the slice or slab. When the second phase tellurium is then removed or annihilated by a further post-annealing step as discussed in more detail in Ser. No. 564,953 filed Dec. 23, 1983, wherein the second post-anneal step is provided in a saturated mercury atmosphere at about 280.degree. C. for a period of from about one up to about four weeks or longer to remove the excess second phase tellurium, and then redistribute residual acceptor impurities throughout the alloy material. The net result of these steps is to provide an alloy which is P-type with an acceptor concentration in a range of about 1-10.times.10.sup.15 /cm.sup.3.
A fundamental barrier to obtaining extrinsically doped P-type material with acceptor concentrations below about 10.sup.15 /cm.sup.3 is the reduction of the residual acceptor impurity concentration. It is therefore necessary that a technique be provided which will result in the desired lower concentrations of impurity. To date, this problem has not been resolved satisfactorily. While it is recognized that the core region of the mercury cadmium telluride slice provides a gettering function to eliminate or minimize these impurities from the device region near the surface, it may also getter a desired dopant impurity. Therefore, gettering of the residual impurities must occur and be removed from the material prior to introduction of the dopants.